Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. The exposed plate is usually developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5 516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675 and 6,482,571).
Conventionally, the plate is exposed with an actinic light (usually an ultraviolet light from a lamp) through a separate photomask film having predetermined imaging pattern which is placed between the light source and the plate. While capable of providing plate with superior lithographic quality, such a method is cumbersome and labor intensive.
Laser sources have been increasingly used to imagewise expose a printing plate which is sensitized to a corresponding laser wavelength. This allows the elimination of the photomask film reducing material, equipment and labor cost. Suitable lasers include infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm) visible lasers (such as frequency-doubled NdYAG laser of about 532 nm, violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 350 to 370 nm). Among them, infrared laser diode, violet laser diode, and ultraviolet laser diode are most attractive.
Laser sensitive plates generally have higher sensitivity (than conventional film based plate) because of the limited laser power and the desire for fast imaging speed. Accordingly, photosensitive plates designed for laser imaging generally have limited room light stability. For example, before being developed to remove the non-hardened areas, frequency-doubled NdYAG laser sensitive plates usually require red room light for handling, violet laser sensitive plates usually require orange or yellow room light for handling, and infrared laser sensitive photopolymer plates usually require yellow room light for handling and have only limited white light stability (due to, for example, the use of certain initiator which has spectral sensitivity in the ultraviolet region).
Such limited room light stability is an inherent barrier for the design and use of laser sensitive lithographic plates. It would be desirable if the exposed laser sensitive plate can be handled under white room light before development or printing.